The present invention relates generally to an electrochemical cell and the use of the electrochemical cell. More specifically, the present invention relates to an improved electrochemical cell that is useful in selectively removing oxygen from a gas stream without decomposing NOx and particularly useful when employed in a NOx sensor.
Depending on how they are run, internal combustion engines that use air as an oxidant are going to produce some amount of NOx. Large amounts of NOx emitted from internal combustion engines have proven to be a significant hazard to human health and to the quality of the environment. At the same time, the near ubiquitous presence of internal combustion engines throughout the industrial world insures that they will be a significant feature of every modern industrial society for at least the immediate future. Thus, if human health and environmental quality are to be preserved, internal combustion engines must be engineered to minimize the amount of NOx emissions.
To compel manufacturers to engineer their products to protect human health and environmental quality, regulatory agencies have imposed standards for NOx emissions that set forth the maximum levels that are permissible from a particular source. As more and more internal combustion engines are operated in ever smaller geographical boundaries, these standards are destined to grow more and more stringent if the overall goal of pollution reduction is to be achieved. To meet these standards, gasoline and diesel engine manufactures must build engines with the ability to adjust their own operation, to optimize the exhaust gas mix. Key to this ability is real time knowledge of the amount NOx being generated by the engine. Thus, the development of improved NOx sensors is critical to developing engines which can meet evolving, tough emissions standards that are in turn key to a healthy environment.
Due to their sensitivity, thermal stability, the ease with which they may be tested and manufactured, and the developed base technology, compact solid electrolyte NOx sensors are of particular interest to designers. Compared to nitrate-based metal salts or binary systems as an auxiliary phase, solid oxide electrolytes bring in high chemical and thermal stability.
A typical sensor utilizing a solid electrolyte consists of a two serial chambered system. The exhaust gas from an internal combustion engine (NO, NO2, CO, CO2, SO2, O2, hydrocarbons, etc.) enters the first chamber configured as an oxygen pump and the coexisting oxygen is removed via the YSZ solid electrolyte according to equation (1):
O2+4exe2x88x92=2O2xe2x88x92xe2x80x83xe2x80x83(1) 
The remainder of the gas diffuses into a second chamber where the NOx is decomposed electrochemically to N2 and O2xe2x88x92 on another electrode according to equations (2) and (3):
NO2+2exe2x88x92=NO+O2xe2x88x92xe2x80x83xe2x80x83(2) 
2NO+4exe2x88x92=N2+2O2xe2x88x92xe2x80x83xe2x80x83(3) 
The current drawn in the second chamber is proportional to the amount of NOx since the only oxygen, in theory, passing through the electrolyte is formed during NOx decomposition. Therefore, there is a great need for a first-chamber electrode, which is capable of catalyzing oxygen reduction (reaction 1) without catalyzing NOx decomposition (reactions 2 and 3). The desire to improve the operation of such systems creates a need for an electrochemical cell that is capable of catalyzing oxygen reduction (reaction 1) without catalyzing NOx decomposition (reactions 2 and 3). As will be apparent to those having skill in the art, such a material is generally desirable in any system wherein it is desirable to remove oxygen from a gas stream containing oxygen and NOx and is particularly desirable as an electrochemical cell utilized in the first chamber of a two chamber NOx sensor that utilized solid electrolyte electrochemical cells.
Accordingly, the present invention is broadly drawn to an electrochemical cell and the use of the electrochemical cell to selectively remove oxygen from a gas stream without decomposing NOx. This aspect of the present invention provides particular utility when the present invention is employed in a NOx sensor. More particularly, an especially useful aspect of the present invention is found when the present invention is employed in a two chambered NOx sensor utilizing solid electrolyte electrochemical cells, wherein the present invention is employed as forming at least a part of the surface of the first chamber, thereby providing an electrochemical cell on the surface of the first chamber capable of catalyzing oxygen reduction without catalyzing NOx decomposition.
The electrochemical cell consists of a solid oxide electrolyte sandwiched between a working, or positive, electrode and a counter, or negative, electrode. The working electrode consists of La1-XMXFeO3, where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5, but preferably is about 0.2. The counter electrode may be any conductive material, but is preferably is a metal stable in air at the operating temperatures of the electrochemical cell. The electrolyte is an oxide with sufficient oxygen ion conductivity, preferably an MOX-stabilized zirconia, where M is Y, Sc, Yb or Ca. By applying a current to the working electrode, and placing it in contact with a gas containing oxygen and NOx, oxygen is selectively reduced (equation 1) and thereby removed from the gas stream through the electrolyte, while NOx concentration is unchanged.
Accordingly, it is an object of the present invention to provide a method of selectively removing oxygen from a gas stream containing NOx and oxygen by contacting the gas stream with an electrochemical cell made from an electrode consisting of La1-XMXFeO3, (where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5), wherein the electrode is on one side of a solid oxide electrolyte, and a counter electrode is on the opposite side of the solid oxide electrolyte, and applying a voltage to the electrochemical cell.
It is a further object of the present invention to provide the counter electrode as a metal.
It is a further object of the present invention to provide the counter electrode as a metal stable in air at the operating temperatures of the electrochemical cell.
It is a further object of the present invention to provide an electrochemical cell for selectively removing oxygen from a gas stream containing NOx and oxygen as an electrode consisting of La1-XMXFeO3, (where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5), and the electrode is affixed to one side of a solid oxide electrolyte, and a counter electrode affixed to the opposite side of the solid oxide electrolyte.
It is a further object of the present invention to provide an improved two chambered NOx sensor utilizing solid oxide electrolyte electrochemical cells, wherein an electrochemical cell capable of catalyzing oxygen reduction without catalyzing NOx decomposition is integral to the first chamber.
It is a further object of the present invention to provide an improved NOx sensor having an electrochemical cell fashioned of a porous electrode consisting of La1-XMXFeO3, (where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5,) the electrode being affixed to one side of a solid oxide electrolyte, and a counter electrode affixed to the opposite side of the solid oxide electrolyte.
These and other objects of the present invention are particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.